Diverse biological sources of core and intact polar isoprenoid GDGTs in terrace soils from southwest of China: Implications for their use as environmental proxies

Archaea are important participants in soil biogeochemical cycles. Their membrane lipids are usually composed of isoprenoid glycerol dialkyl glycerol tetraethers (iGDGTs), which are ubiquitous in the environments and widely applied as biomarkers for archaeal biogeochemical processes and palaeothermometry. However, the biological sources of iGDGTs and environmental factors controlling their distribution in soils are elusive. We examined the archaeal lipid composition and community structure along an altitudinal soil transect in the Hani terrace wetlands in Southwest of China by combining organic geochemical and high-throughput sequencing approaches. The results show that monoglycosyl-iGDGTs (1G-iGDGTs) are the dominating intact polar lipids (IPLs) and possible precursor of core-iGDGTs preserved in soils. iGDGTs from Hani soils may come from several archaeal lineages based on their significant positive correlations in both core lipids and IPLs. Thaumarchaeota Group I.1b most likely contribute to crenarchaeol and its regio-isomer in non-paddy soils, while Bathyarchaeota, Methanobacteria and Methanomicrobia may predominantly contribute to GDGT-0 in water-saturated paddy soils. Our results indicate that the paleotemperature proxy TEX86 in soils can be affected by changes in archaeal community structure in addition to environmental variables such as soil water content and pH, but not independently by temperature. In addition, the lower TEX86 in humid soils may be partly attributed to the insufficient peak separation of iGDGTs with cyclopentyl moieties, which contain abnormally high abundances of shoulder peaks that may be contributed by anaerobic Archaea. This study sheds light on the increasing role of microbial community structure in affecting the fidelity of TEX86 in terrestrial environment, which should be taken into consideration when applying it to paleo-soil temperature studies.